1. Field of the Invention
This invention relates to a method of making goethite powder and more particularly to a method of making goethite powder useful for making maghemite powder of high coercivity.
2. Description of the Prior Art
In a magnetic tape suitable for use with a tape recorder, a video tape recorder and the like, acicular maghemite powder (or gamma-Fe.sub.2 O.sub.3 powder) is widely used. However, as the coercivity (Hc) of conventional acicular maghemite powder is under 400 oersteds, it is not suitable for high density recording.
Acicular maghemite powder made by the known route of reducing and oxidizing goethite powder (or alpha-FeOOH powder) has a coercivity which depends on the particle size, the particle shape, and the particle size uniformity of the goethite powder used as the starting material.
Heretofore, two methods have commonly been employed for making conventional goethite powder. One of these methods involves bubbling air through a solution of iron sulfate and sodium hydroxide to form goethite powder and sulfuric acid, and then the resulting sulfuric acid is changed to iron sulfate by iron. One disadvantage of this method is that excessive time (typically ranging from several days to several months) are required for the formation of goethite. Another disadvantage of this method is that the coercivity (Hc) of the maghemite powder derived from the resulting goethite by the above indicated known route, is under 370 oersteds.
The other of these methods involves bubbling air though an aqueous solution of ferrous salt and alkaline hydroxide to form goethite powder. The maghemite powder derived from such resulting goethite by the above indicated known route generally has a higher coercivity (Hc) than the maghemite powder derived from the goethite obtained by the preceding two-stage method. However, one disadvantage of the second method is that about 10 to 20 hours are required for the formation of goethite. Another disadvantage of this second method is that the hydroxide is apt to gel and be non-uniform, particularly if a suspension of ferrous hydroxide of high density is used, thereby making it is very difficult to form a geothite powder with uniform particle size.